In medium-sized skillet, melt butter and brown sugar over medium heat. Add onions and salt; cook until softened, about 5 minutes. Add beer and reduce heat to medium low. Cover and simmer 20 minutes. Remove lid and continue cooking at light simmer until liquid is thickened and mixture is medium to dark golden brown and onions caramelize, about 40 minutes. Remove from heat and set aside.
Butterfly brats lengthwise about 3/4 of the way through and split open so that the brat remains in one piece. In skillet over medium heat, sear brats until each side is golden brown. Set aside.
Butter outside of each slice of bread. Divide mustard and spread on the inside of 4 bread slices. On inside of remaining 4 slices, divide and add limburger. On top of limburger, layer 1/4 of the caramelized onions, a brat and 1 baby swiss cheese slice; top each sandwich with remaining pieces of bread, mustard-side-down.
Brown sandwiches on preheated griddle pan or in skillet over medium-low heat until each side is golden brown and cheese is melted, flipping once. Once browned, let sandwiches rest for about 1 minute. Slice in half before serving.
- 1 batch beer caramelized onions (directions follow)
- 1/2 tablespoon butter
- 2 tablespoons brown sugar, packed
- 1 large sweet onion, cut in 1/4-inch-thick slices
- 1/2 teaspoon kosher salt
- 1/2 cup bock beer
- 4 fully cooked beer brats
- 3-4 tablespoons butter, softened
- 8 slices pretzel bread, cut 3/4-inch-thick
- 3 tablespoons whole grain mustard
- 3 ounces Wisconsin Limburger cheese
- 4 1-ounce slices Wisconsin Baby Swiss cheese, sliced